Induction Motor Tests Quiz

Objectives of the Tests

• Draw the circle diagram and obtain torque-slip characteristics for slip ranging from 0 to 1.
• Plot current, torque, power factor, efficiency, and slip against output characteristics for both stable and unstable regions.
• Determine the line current, power factor, and slip at full load output using the circle diagram.
• Predetermine maximum torque and corresponding slip from the circle diagram.
• Obtain the rotational speed when the motor operates as a generator using the circle diagram.

Equipment Required

• Motor: 3-phase squirrel cage induction motor, 5HP, 7.3A, 1440 RPM, 50 Hz, 400/440 V.
• Ammeter: MI 0-5A, 0-10A; MC 0-5A.
• Voltmeter: MI 0-600V, 0-150V; MC 0-15V.
• Wattmeter: 150V, 10A (upf); 500V, 5A (ipf).
• Other: 1002, 5A rheostat.

Principle of Testing

• No-load and blocked rotor tests provide data to calculate induction motor equivalent circuit parameters.
• Equivalent circuit behaves like a series R-L circuit with constant reactance and variable resistance.
• Current locus resembles a semi-circle, with the diameter indicating maximum current.

Procedure

• No-Load Test:
  • Connect motor through an auto transformer starter.
  • Gradually apply rated voltage to minimize starting current.
  • Obtain steady-state no-load readings noted in a table.
• Blocked-Rotor Test:
  • Connect the motor via an auto transformer and physically block the rotor.
  • Gradually increase voltage until rated current is reached.
  • Record meter readings and measure winding resistance.

Data Processing

• Draw the circle diagram using required data: ( I_o, I_{sc}, I_{2}' ) and se.
• ( I_{2}' ) (blocked rotor current at rated voltage) is calculated with ( I_{se} \times \text{Rated phase voltage} ).
• Choose a suitable current scale (e.g., 1 cm = 1 ampere).
• Power scale defined as ( 1 \text{ cm} = 3 \times V_{ph} \times \text{Current scale} ).

Circle Diagram Construction

• Plot the current vector ( OA ) at angle ( d ) to Y-axis with length equivalent to current scale.
• Draw vector ( OB ) at angle ( se ) such that length matches ( I_{2} ) to scale; join points A and B.
• Construct a perpendicular bisector to line AB, meeting the horizontal line through A at point D; draw a semi-circle with radius DA.
• Drop a vertical from B to line AD to establish point E, and select point F on BE for rotor and stator copper loss ratio.
• Define output line AB and torque line AF; plot points ( L_{i} ) on the semi-circle and calculate respective values for torque, power loss, efficiency, and slip.
• Analyze maximum torque by drawing a tangent to the torque line that contacts the circle; this gives maximum torque and corresponding slip.

Results

• Successfully draw the circle diagram and plot performance characteristics based on the data obtained, providing detailed insights into the motor's behavior under different loads.